1. Field of the Invention
This invention concerns a process for removing sulfur from naphtha, a petroleum product used to make fuels like gasoline. Specifically, deactivated hydrotreating catalyst is used to selectively hydrodesulfurize naphtha while minimizing olefin loss.
2. Description of Related Information
It is well known that air pollution is a serious environmental problem. A major source of air pollution worldwide is the exhaust from hundreds of millions of motor vehicles due to fuel combustion. Laws and regulations have been enacted reflecting the need to reduce harmful motor vehicle emissions through more restrictive fuel standards. Fuels containing sulfur produce sulfur dioxide and other pollutants leading to a host of environmental concerns, such as smog and related health issues, acid rain leading to deforestation, water pollution, as well as other environmental problems. To help reduce or eliminate these environmental problems, the sulfur content of fuels has been, and will continue to be, restricted to increasingly smaller concentrations, such as less than 100 or even 50 parts per million (ppm).
The problem of sulfur in fuels is compounded in many areas where there is diminishing or no domestic source of crude oil having relatively low sulfur content. For example, in the United States the supply of domestic oil production relies increasingly on lower grade crude oil with higher sulfur content. The need for lower sulfur content fuel therefore increases demand for imported oil having lower sulfur content increasing trade imbalance and vulnerability due to dependence on foreign sources of oil.
The sulfur content in crude oil can take the form of a wide variety of both aliphatic and aromatic sulfurous hydrocarbons. Various techniques have been developed for removing sulfur compounds. One such technique, called hydrodesulfurization (HDS), involves catalytically reacting hydrogen with the sulfur compounds. The general HDS reaction is illustrated in Equation 1. EQU RSR'+H.sub.2 .fwdarw.RH+R'H+H.sub.2 S EQU Equation 1: Hydrodesulfurization Reaction
In Equation 1, the sulfur compound, RSR', may be: a thiol or mercaptan, where R is hydrocarbyl and R' is hydrogen; a sulfide or disulfide, where the sulfur is connected to another sulfur atom in R or R' hydrocarbyl groups; or may be a thiophene where R and R' are connected to form a heterocyclic ring. The HDS reaction consumes hydrogen (H.sub.2) and produces hydrogen sulfide (H.sub.2 S) and hydrocarbons wherein the sulfur atom is replaced by two hydrogen atoms. The hydrogen sulfide can then be separated to give a petroleum product in which the sulfur is significantly reduced or substantially eliminated.
HDS is one process within a class of processes called hydrotreating, or hydroprocessing, involving the introduction and reaction of hydrogen with various hydrocarbonaceous compounds. General hydrotreating reactions with oxygen compounds, nitrogen compounds and unsaturated hydrocarbons, including olefins, are illustrated in Equations 2, 3 and 4, respectively. EQU ROR'+H.sub.2 .fwdarw.RH+R'H+H.sub.2 O EQU Equation 2: Hydrodeoxygenation Reaction ##STR1##
The hydrotreating reactions can occur simultaneously to various degrees when sulfur-, oxygen-, nitrogen-containing and unsaturated compounds are present in the petroleum. The hydrotreating reactions are exothermic, producing heat. Such hydrotreatment has been used to remove not only sulfur, but to also remove nitrogen and other materials, like metals, not only for environmental considerations but for other uses, such as to protect catalysts used in subsequent processing from being poisoned by such elements. See, for example, Applied Industrial Catalysis, Volume I, edited by B. E. Leach, Academic Press (1983); Chemistry of Catalytic Processes, by B. C. Gates et al., McGraw-Hill (1979); and Applied Heterogeneous Catalysis: Design Manufacture Use of Solid Catalysts, by J. F. LePage et al., Technip, Paris (1987).
Olefins are useful in fuel feedstock by raising the octane number of the fuel, increasing its value and performance properties. For example, cracked naphtha typically contains over 20 weight percent olefins having octane numbers that are higher than the corresponding saturated hydrocarbons. HDS of naphtha using standard hydrotreating catalysts under conditions required for sulfur removal produces a significant loss of olefins through hydrogenation. This produces a lower grade fuel which then needs more refining, such as isomerization, blending, or other refining, to produce higher octane fuel, adding significantly to production expenses.
Selective HDS to remove sulfur while minimizing hydrogenation of olefins and octane reduction by various techniques, such as selective catalysts, have been described. For example, U.S. Pat. Nos. 4,132,632 (Yu et al.) and No. 4,140,626 (Bertolacini et al.) disclose selective desulfurization of cracked naphthas by using specific catalyst having particular amounts of Group VIB and VIII metals on magnesia support. U.S. Pat. No. 4,149,965 (Pine et al.) discloses a process for starting-up naphtha HDS using partially deactivated hydrotreating catalyst under relatively low pressure of less than 200 psig. The catalyst is partially deactivated using a substantially non-metals containing, hydrocarbonaceous oil for a time ranging from about 10 hours to about 20 days. U.S. Pat. No. 2,983,669 (Noll) discloses processes for treating petroleum with high sulfur content using fractionation and HDS. Noll suggests that the HDS catalyst may be one which does not readily promote hydrogenation, such as a partially spent catalyst.
Hydrotreating catalysts age, losing activity during use by collecting deposits of carbonaceous material and/or impurities, such as metals, from the treated feedstock. Eventually, with increased deposition the catalyst is no longer able to provide effective hydrotreating. The deactivated catalyst may be regenerated. The regenerated catalyst can be reused but is generally less effective than fresh catalyst by requiring higher temperature to give the desired activity and becoming deactivated more quickly than fresh catalyst. Although hydrotreating catalysts can usually be repetitively regenerated, they eventually become irreversibly deactivated, or spent, essentially losing their intended hydrotreating utility.
Spent hydrotreating catalysts have been used in hydrotreating, including HDS. For example, U.S. Pat. No. 3,876,532 (Plundo et al.) discloses a process for hydrotreating middle distillate, virgin oils using spent hydrotreating catalyst under extremely mild conditions to reduce acid and mercaptan content, to remove sulfur below 0.2 weight percent, or 2,000 ppm. U.S. Pat. No. 4,414,102 (Rankel et al.) discloses the use of spent HDS catalyst to transform nitrogen- or oxygen-containing compounds to sulfur-containing compounds followed by mild HDS treatment.
It would be desirable to have a process for removing sulfur from fuel feedstocks, like naphtha, containing olefins which minimizes loss of octane value using an inexpensive procedure under a wide range of conditions, to provide a cleaner environment along with a more stable economy.